Preparation and characterization of Langmuir-Blodgett films of a geminally branched lipid acid have been achieved for the first time. Pressure-area isotherms of cadmium α,α-dimethylarachidate (2,2-dimethyl-1-eicosanoate) exhibit two condensed phases, one at an area consistent with a single layer expanded somewhat from cadmium arachidate by the methyl groups, the other at a much smaller area which could be consistent with trilayer formation. The isotherms are quite distinct considering the discontinuity in packing expected a priori by the presence of the geminal dimethyl substitution. Transfer to a solid surface from the air-water interface was accomplished at the less condensed phase and the resulting films were characterized as a monolayer, slightly expanded relative to cadmium arachidate. This characterization was made by a combination of two techniques: attenuated total internal reflectance shifts in the excitation of surface plasmons and electron mean free path of escape from X-ray photoelectron spectroscopy; both techniques were required as neither alone gave complete understanding. Transfer at the more condensed phase produced a layer somewhat thicker than that produced at the less condensed phase, but not as thick as expected from film area; expansion upon transfer apparently occurred, and this prevents full characterization. However, the more condensed phase is reproducible and is apparently a stable phase. Preparation of the substituted arachidic acid by an unambiguous route and characterization of the monolayer assemblies using the two techniques are discussed. © 1983 American Chemical Society.